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Title: Fusion-fission of {sup 16}O+{sup 197}Au at sub-barrier energies.

Abstract

The recent discovery of heavy-ion fusion hindrance at far sub-barrier energies has focused much attention on both experimental and theoretical studies of this phenomenon. Most of the experimental evidence comes from medium-heavy systems such as Ni+Ni to Zr+Zr, for which the compound system decays primarily by charged-particle evaporation. In order to study heavier systems, it is, however, necessary to measure also the fraction of the decay that goes into fission fragments. In the present work we have, therefore, measured the fission cross section of {sup 16}O+{sup 197}Au down to unprecedented far sub-barrier energies using a large position sensitive PPAC placed at backward angles. The preliminary cross sections will be discussed and compared to earlier studies at near-barrier energies. No conclusive evidence for sub-barrier hindrance was found, probably because the measurements were not extended to sufficiently low energies.

Authors:
; ; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
947978
Report Number(s):
ANL/PHY/CP-118623
TRN: US0901488
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Conference
Resource Relation:
Conference: International Conference on Reaction Mechanisms and Nuclear Structure at the Coulomb Barrier (Fusion 06); Mar. 19, 2006 - Mar. 23, 2006; San Servolo, Venezia, Italy
Country of Publication:
United States
Language:
ENGLISH
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; COULOMB FIELD; CROSS SECTIONS; DECAY; EVAPORATION; FISSION; FISSION FRAGMENTS; NUCLEAR STRUCTURE; REACTION KINETICS

Citation Formats

Back, B. B., Jiang, C. L., Janssens, R. V. F., Henderson, D. J., Shumard, B. R., Lister, C. J., Peterson, D., Rehm, K. E., Tanihata, I., Tang, X., Wang, X. F., Zhu, S., and Physics. Fusion-fission of {sup 16}O+{sup 197}Au at sub-barrier energies.. United States: N. p., 2006. Web.
Back, B. B., Jiang, C. L., Janssens, R. V. F., Henderson, D. J., Shumard, B. R., Lister, C. J., Peterson, D., Rehm, K. E., Tanihata, I., Tang, X., Wang, X. F., Zhu, S., & Physics. Fusion-fission of {sup 16}O+{sup 197}Au at sub-barrier energies.. United States.
Back, B. B., Jiang, C. L., Janssens, R. V. F., Henderson, D. J., Shumard, B. R., Lister, C. J., Peterson, D., Rehm, K. E., Tanihata, I., Tang, X., Wang, X. F., Zhu, S., and Physics. Sun . "Fusion-fission of {sup 16}O+{sup 197}Au at sub-barrier energies.". United States. doi:.
@article{osti_947978,
title = {Fusion-fission of {sup 16}O+{sup 197}Au at sub-barrier energies.},
author = {Back, B. B. and Jiang, C. L. and Janssens, R. V. F. and Henderson, D. J. and Shumard, B. R. and Lister, C. J. and Peterson, D. and Rehm, K. E. and Tanihata, I. and Tang, X. and Wang, X. F. and Zhu, S. and Physics},
abstractNote = {The recent discovery of heavy-ion fusion hindrance at far sub-barrier energies has focused much attention on both experimental and theoretical studies of this phenomenon. Most of the experimental evidence comes from medium-heavy systems such as Ni+Ni to Zr+Zr, for which the compound system decays primarily by charged-particle evaporation. In order to study heavier systems, it is, however, necessary to measure also the fraction of the decay that goes into fission fragments. In the present work we have, therefore, measured the fission cross section of {sup 16}O+{sup 197}Au down to unprecedented far sub-barrier energies using a large position sensitive PPAC placed at backward angles. The preliminary cross sections will be discussed and compared to earlier studies at near-barrier energies. No conclusive evidence for sub-barrier hindrance was found, probably because the measurements were not extended to sufficiently low energies.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}

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  • Angular distribution of fission fragments have been measured for {sup 19}F+{sup 197}Au reaction at bombarding energies from 91 to 110 MeV. Fission fragment angular distributions have been calculated by transition state model with the transmission coefficients obtained using the coupled-channels theory. The calculated angular anisotropies are in good agreement with the experimental anisotropies. The experimental fission cross sections have also been reproduced on the basis of the coupled-channels theory. The results of angular distribution measurement do not show any significant contribution from quasifission as was reported in the literature based on the measurement of evaporation residues and mass distribution.
  • With a view to extending measurements of the fission probability into the deep sub-barrier region, the fission probability into the deep sub-barrier region, the fission cross-section for {sup 12}C+{sup 197}Au and the thick target fission yield for {sup 12}+{sup 237,238}U have been measured at projectile energies from 80 to 50 MeV. The fission fragments were identified by the energy - time-of-flight method employing a bunched beam. For {sup 12}C+{sup 197}Au an upper limit of 2 {mu}b was established for the cross-sections at 50 MeV bombarding energy. The thick target yield measurements at 50 MeV gave upper limits of about 10more » {mu}b and 80{mu}b for {sup 238}U and {sup 235}U respectively, limited by neutron induced background fission.« less
  • Using the fragment folding angle technique, the complete fusion-fission is separated from the transfer-induced fission. The cross sections of the complete fusion-fission and fragment angular distributions for the systems of [sup 16]O+[sup 232]Th, [sup 19]F+[sup 232]Th, and [sup 16]O+[sup 238]U at near- and sub-barrier energies have been measured. The observed fission excitation functions can be fitted very well by coupled-channels theory calculations. It is found that fragments from complete fusion fission show a smaller angular anisotropy as compared with our previous measurements in which the transfer-induced fission component was not excluded. However, the measured angular anisotropies of complete fusion-fission aremore » obviously greater than expected on the basis of theoretical models in which the effect of prefission neutron emission was taken into account. Also the peak in the variation of the anisotropy with incident energy still persists in the [sup 19]F+[sup 232]Th case.« less
  • Proton and [alpha]-particle spectral shapes and multiplicities have been measured in coincidence with evaporation residues from 145 to 220 MeV [sup 28]Si+[sup 165]Ho and 115 and 140 MeV [sup 16]O+[sup 197]Au,[sup 208]Pb fusion reactions. Evaporation residues were separated using an electrostatic deflector and detected with large area surface barrier detectors. Light charged particles were detected at forward and backward angles with 14 single NaI detectors. In the context of the statistical model, the charged particle spectra provide information about the shapes and level densities of the emitting systems. Deformed emitters are inferred, and to a varying degree, an energy-dependent levelmore » density parameter is compatible with the data in each of the three cases. Implications for current fusion and fission dynamics studies are discussed.« less
  • The fusion of the heavy asymmetric systems of /sup 16/O, /sup 18/O+ /sup 208/Pb and /sup 15/N, /sup 16/O+ /sup 209/Bi was studied at bombarding energies near and below the Coulomb barrier. The fusion-fission, transfer, and elastic angular distributions were measured for all four systems. The evaporation residue component of the fusion was measured for only the /sup 16/O+ /sup 208/Pb..-->.. /sup 224/Th reaction; in agreement with statistical model calculations, the cross section for the evaporation residues was found to be negligible compared to the fission, and hence can be ignored in all the systems. The shapes of the fissionmore » fragment angular distributions are then a direct measure of the entrance channel spins which lead to fusion. The experimental sub-barrier fusion cross sections are underpredicted by the one-dimensional barrier penetration model in all cases. However, if the zero point motion of the targets is included in the barrier penetration model, the energy dependence of both the total fusion and the fission angular distributions can be explained for the /sup 15/N+ /sup 209/Bi system alone. To fit the low-energy fusion of the oxygen reactions requires considerably more zero point motion: this being inconsistent with the fact that this effect should be the same in all the reactions. Also, the cross sections measured for the transfer channels were found to exhibit the same trend as the fusion at the lower energies. This correlation suggests that some of the sub-barrier fusion in the oxygen-induced reactions may be due to transfer-related processes.« less